One of the methods to achieve tracking servo when performing recording and reproducing of an optical disk is a differential phase detection (DPD) method (for example, see Japanese Patent Application Laid-open No. 2001-216663). The DPD method uses a diffraction by a code (a recording mark) on a recording layer of a disk, in which a quadrant optical detector 100 as shown in FIG. 18 is used as an optical detector for receiving an optical beam reflected from a recording surface.
A light receiving surface of the optical detector 100 is arranged at a position to receive an optical beam, and as shown in FIG. 18, divided into four square-shaped areas (light receiving areas A to D) of the same dimension. The optical detector 100 outputs a signal having an amplitude of a value (a received light amount) obtained from a surface integral of an intensity of the optical beam over a light receiving surface for each of the light receiving areas.
In the DPP method, two phase difference signals P(IA, IB) and P(IC, ID) are obtained from an output signal of the optical detector 100, where IX is an output signal corresponding to a light receiving area X and P(X, Y) is a function representing a phase difference between a signal X and a signal Y. A signal which is obtained by passing a sum signal P(IA, IB)+P(IC, ID) obtained by adding the two signals through a low pass filter serves as a tracking error signal (a DPD signal) in the DPD method. Alternatively, a signal which is obtained by passing a signal P(IA+IC, IB+ID) through the low pass filter can be used as the tracking error signal.